1. Field of the invention
The invention is in the field of measuring systems with respect to the quality of optical transmission systems. It relates to an optical circuit for a measuring system for measuring the sensitivity of an optical transmission system involving reflections and Rayleigh backscattering, and a measuring system of the said type provided with such an optical circuit.
2. Prior art
An optical transmission line in a passive optical network, as implemented, for example, by a glass fibre, is suitable for the transmission of optical signals in two directions, which makes it unnecessary, in principle, to provide a separate line for the return path. In the case of bidirectional use there is, however, the risk of crosstalk between the optical signals in the two directions, owing to the discrete reflections and reflections by Rayleigh backscattering, as a result of which the performance of the optical transmission system in which the transmission line is incorporated, is adversely affected. Such reflections lead to receiver sensitivity degradation and represent an additional burden on the total optical power available in the optical network. This additional burden is sometimes denoted by the term crosstalk penalty. Consequently, this must be taken into account when optical networks are being designed and dimensioned, in order to prevent as far as possible, subsequent undesirable system degradation in implemented networks. For this purpose, measuring systems are known which incorporate characteristic elements of the optical network to be designed, and by means of which the reception sensitivity and the crosstalk penalty are subsequently measured. Thus reference [1] describes a measuring arrangement which is used specifically for measuring the reception sensitivity due to Rayleigh backscattering in the form of bit error probabilities and the associated crosstalk penalty in an optical communication system having two signal sources of CPFSK-modulated signals. This measuring arrangement is based on an optical circuit containing two 3 dB couplers between which a monomodal glass fibre of a certain length (typically 1 km) is incorporated, a first signal source being connected to a connection point of the first coupler, and the second signal source and the receiving means being connected to two connection points of the second coupler. The remaining connection points of the couplers are not used. The connections to the couplers are of such a type that a signal emanating from the first signal source can arrive at the receiving means (at least mainly) via the glass fibre in a forward signal direction, while a signal emanating from the second signal source can arrive at the receiving means via the glass fibre only (at least mainly) as a backscattered signal. Incorporated between each of the signal sources and the respective connection point of the coupler, to which the signal source is connected, there is a controllable optical signal attenuator for controlling the signal intensity of the generated signal. The connection points are chosen so as to be reflection-free to a high degree. This known measuring arrangement for measuring the effect of Rayleigh backscattering in a length of glass fibre has a number of drawbacks, however. A first drawback is that the signal sources are in fact coupled directly via the couplers, to one another in the forward signal direction; consequently, not only can optical feedback occur in each of the signal sources owing to backscattering effects, but also to direct signal infeed. The coupling of the signal sources to the couplers therefore requires a particularly large isolation. A second drawback is that the signal emanating from the signal source which is coupled to the receiving means in the forward signal direction, likewise must pass through the entire length of the glass fibre. In the process, the last-mentioned signal undergoes additional attenuation, and dispersion and double backscattering occur, with unclear effects on the total signal arriving at the receiving means, as a result of which a measurement is no longer reliable. A third drawback is the following. The most disadvantageous effects on the reception sensitivity occur if the backscattered signal were to be completely polarized. In the known measuring arrangement it is not, however, possible to measure the effect of a completely polarized backscattered signal without affecting the signal which emanates from the signal source which is coupled to the receiving means in the forward signal direction. Reference [2] discloses an optical circuit which is suitable for a measuring arrangement which can be used to measure the effect of a discrete reflection element. This optical circuit comprises a 3 dB coupler having four connection points and the reflection element connected to one of the four connection points, while the remaining connection points are designed for the connection of one or two signal sources and receiving means. In a first measuring arrangement, a first signal source and the reflection element in the forward signal direction are coupled to the receiving means, and signals emanating from the second signal source can only arrive at the receiving means via the reflection element. Incorporated between the reflection element and its connection to the coupler there is a controllable signal attenuator. Said first measuring arrangement based on this known optical circuit likewise has the drawback that the two signal sources are coupled to one another in the forward signal direction, so that additional isolating measures are required to prevent adverse effects on the signal generation. This measuring arrangement also has the drawback that the effect of a completely polarized reflected signal cannot be measured without affecting the signal emanating from the signal source which is coupled to the receiving means in the forward signal direction. In a second measuring arrangement, in which only one signal source is used as the transmitter, which is coupled both to the reflection element and to the receiving, or, as the case may be, detection means in the forward signal direction, it is precisely the adverse effects of reflections on the signal generation which can be measured. Measuring arrangements according to the prior art disclosed by references [1] and [2] are only of limited use as it is not stated how the effect of a finite number of discrete reflection elements, in combination or not in combination with Rayleigh backscattering, can be measured. An optical network, implemented in practice, of some size will in general contain a large number of discrete reflection points. Signals emanating from a number of such discrete reflection points may mutually interfere, however, and thus result in greater system degradation than a signal emanating from a single reflection point. Since Rayleigh backscattering may be regarded as reflection at an infinite number of discrete reflection points, the measurement thereof could be used for assessing the consequences of a finite, large number of discrete reflection points. This, however, is possible to a limited extent since, as disclosed, for example, by reference [3], cumulative Rayleigh backscattering does not increase with length beyond a certain length which is a function of the wavelength employed of the optical signal.
In the light of the said drawbacks of the cited prior art, it can be assumed that there is a need for a novel or improved optical circuit for a measuring system of the type indicated, in which all the effects on which the reflection sensitivity of an optical transmission system depends can be influenced, as far as possible separately and to a sufficient degree, and in which signal sources to be connected are not coupled in the forward signal direction.